There isn't, no.  It will already be in place, and once you start to pull shards the counter will increase according to the text in the game.  Unfortunately you won't see anything visual to represent it as the entire thing is in the background, but after xx shard pulls without a legendary the odds will increase

I keep a shard pull record so I have a good idea when mercy is due.  

Finally, I am not the last kid on the block.  

And for some reason Plarium won't share that information with it's player base, though they could - you can see how easily when they track when its one of their events like burn 150 ancient to get a legendary

So we have to revert to spreadsheets or some other way of tracking them ourselves

For me, spreadsheets are part of the game.  

I agree that it would be good for Plarium to share the information.  It would be trivial as the number is already there, they just need to show it.  I believe though that it would actually reduce sales, as people would be less likely to spend when they know their mercy is due to hit.  They would also save the mercy for a double legendary event or something.

I imagine their decision is based around revenue - when isn't it!

But I thought about that, FTP don't buy so no harm giving them the info

Marginal spenders may spend less but I'd think more if they were approaching mercy not less

Big spenders probably already use their own system of keeping track

I think the real reason is because Plarium have sorta painted themselves into a corner with all these guaranteed lego events and 10x/2x comboes. If they showed this information publicly, even more people than already do, would be saving up specifically around those to maximize their chances of getting the targeted champs through the mercy system.

Plarium want people to impulsively pull without actually having good chances of getting the rewards, because that increases the chance they continue to spend. Players playing efficiently is the bane of their business model.